Perturbation of intracellular Ca2+ signaling and prolongation of the action potential accompany many forms of heart disease and contribute to the pathogenesis of cardiac hypertrophy and failure. The L-type Ca2+ channel is the major mediator of Ca2+ influx into cardiomyocytes and an important determinant of action potential morphology. The role of the L-type channel - the proximal element in Ca2+ signaling - in the pathophysiology of cardiac hypertrophy is poorly characterized. Calcineurin is a cytoplasmic protein phosphatase that mediates hypertrophic signaling in many models of cardiac hypertrophy including our own model of aortic banding in mice. We have reported that activity and abundance of the L-type Ca2+ channel are altered in pressure-overload cardiac hypertrophy through calcineurin-dependent pathways. In this application, we propose to elucidate the underlying mechanisms. This proposal is based on the hypotheses that calcineurin is a regulatory component of the L-type Ca2+ channel complex in cardiac myocytes. We further hypothesize that calcineurin-dependent mechanisms regulate Ca2+ channel function both directly and through altered gene expression. Finally, we hypothesize that these changes - post-translational and transcriptional - contribute to hypertrophic prolongation of the ventricular action potential and consequent arrhythmogenesis. We have collected evidence that calcineurin is a component of the L-type channel macromolecular complex in heart, a finding which will be confirmed and extended in Aim 1. In Aim 2, we will determine whether calcineurin regulates Ca2+ channel function using suppression and heterologous reconstitution strategies. In Aim 3, we will pursue our observation that calcineurin, in addition to regulating channel activity, also regulates transcription of alpha1c, the channel pore-forming subunit. We have cloned a large, 5' fragment of the alpha1c gene and generated reporter constructs to use in transient transfection experiments. We have also generated "alpha1c reporter" transgenic mice, which we will use to decipher pathways governing a1c expression in vivo. In Aim 4, we will determine whether the mechanisms of calcineurin regulation of Ca2+ channel expression and function examined in Aims 1-3 occur in a clinically relevant, in vivo model of hypertrophy.